1. Field of the Disclosure
The invention relates to a method for regulating the charging state (charging state regulating method) of an energy accumulator for a vehicle with hybrid drive as well as a control unit for carrying out the method. In this way, a driving speed variable value of the vehicle is detected by means of a driving speed detection device and a nominal charging state of the energy accumulator is determined by the control unit as a function of the driving speed variable value. After detecting an actual charging state of the energy accumulator by means of the charging state detection means, the control unit compares the actual charging state with the nominal charging state.
2. Description of Related Art
Such methods are known, for example, from DE 10346213 A1 and DE 44 30 670 A1. Vehicles with hybrid drive have an internal combustion engine and an electric motor. During startup and acceleration or revving up the internal combustion engine, a torque is applied to it by means of the electric motor. This can be carried out, for example, via a coupling of the electric motor to the drive train of the vehicle. When the vehicle is braked, the electric motor is used as a generator, by means of which the kinetic energy of the vehicle (braking energy) is transformed into electrical energy and is stored again in the energy accumulator. In this way, the energy consumption of the motor vehicle can be considerably reduced when compared with a motor vehicle having only an internal combustion engine.
In order to regulate the charging state of the energy accumulator, a function or characteristic curve is defined as a function of driving speed variable values, i.e., of values that are proportional to the driving speed or the driving speed itself, which indicates a lower limit of the charging state of the energy accumulator. If the real-time charging state of the energy accumulator, thus the actual charging state, lies below a nominal charging state determined for a real-time driving speed variable value by means of the function or characteristic curve, then the energy accumulator will be charged up by means of the electric motor used as a generator. Otherwise, a charging up of the energy accumulator is not conducted by the internal combustion engine. The characteristic curve or the function is designed in such a way that continuously lower charging states of the energy accumulator are permitted with increasing real-time driving speed without calling upon the internal combustion engine for charging up the accumulator. This is based on the fact that with increasing driving speed, the probability increases that the energy accumulator will be charged up in a timely manner by a braking and an overcharging of the energy accumulator shall be prevented.
The known methods proceed implicitly from a relatively large accumulator, whose charging state is increased by the braking energy only within a certain range. In addition, they proceed from the fact that a certain charging state of the accumulator must at least be maintained, which is why the internal combustion engine will also be called upon as needed for charging up the accumulator. These methods are thus well suitable for batteries and storage batteries as accumulators. Batteries, however, have a relatively poor efficiency with respect to “charging up the necessary energy” while “discharging reusable energy”, since they have a relatively high internal resistance, which is caused by the chemical reactions in the battery during charging and discharging.
Further, batteries and storage batteries wear out even after a few thousand charging cycles, whereby their efficiency is further adversely affected. It is also known to use capacitors for storing electrical energy. Capacitors have a far better efficiency than batteries or storage batteries and wear out much less rapidly. For example, the efficiency of capacitors is reduced only after the number of charging cycles is in the range of millions. Capacitors, however, are relatively small energy accumulators, that is, their charging capacity is not much greater than the braking energy that occurs when the vehicle is braked from its highest speed to a complete stop. Capacitors thus have only a small charging capacity, i.e., a small maximum energy storage capacity when compared with batteries or storage batteries, so that the known charging state regulating methods are only poorly suitable for regulating the charging state of a capacitor.
Document WO 2007/028755 A1 discloses a device for supplying energy as well as a method for controlling the flow of energy in a hybrid vehicle, which provides two energy accumulators, wherein a charging regulator is present, which adjusts the nominal charging state of one of the energy accumulators in such a way that the nominal charging state of this one energy accumulator is decreased with increasing speed of the hybrid vehicle.